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U.S. fertility rate is on the rise

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The birth rate in the U.S. increased between 2013 and 2014 for the first time since 2007

Trend linked to more women in 30s and 40s having babies as birth rates among younger women dropped

(CNN)The rate of twin births in the United States reached a record high in 2014, according to an annual report released Wednesday by the Centers for Disease Control and Prevention's National Center for Health Statistics. There were 33.9 twins born for every 1,000 births in 2014.

The twin birth rate has been climbing most years since 1980 among white, black and Hispanic women. Between 2013 and 2014, however, it only went up among black women, by 4%, whereas the rates did not change among white and Hispanic women.

Births of triplets and higher multiples were on the rise from 1980 to 1998, when the rate peaked at 193.5 multiples for every 1,000 births. But unlike twin births, the rate has been declining for 20 years. Between 2013 and 2014, it dropped 5% to 113.5 per 1,000 births.

The overall increase in twin and multiple births could be due in part to assisted reproductive technology, of which in vitro fertilization is the most common. Use of the technology has been on the rise since the first baby was conceived using this technology in the United States in 1981.

However, as the authors of the report suggest, changes in assisted reproductive technology could be responsible for recent decreases in the number of triplets and higher multiples. According to a different 2015 CDC report, the number of embryos implanted has decreased among women of all age groups. On average, about two embryos are transferred for women under 35 and women 35 to 40, whereas 2.7 embryos are transferred for women older than 40.

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Couples who don't succeed at first with IVF should try more than three or four cycles, according to another study published Tuesday in The Journal of the American Medical Association.

British researchers followed more than 150,000 women who underwent IVF cycles over a seven-year period. They found the pregnancy rate is nearly 30% for the first cycle, more than 20% for the second through fourth cycle, as high as 65% by the sixth cycle and continues to increase through the ninth cycle. Most couples stop after only a few cycles, in part because of the emotional and financial toll of the process.

Regardless of how a mother becomes pregnant, there are a number of risks associated with twin and multiple pregnancies, including gestational diabetes, high blood pressure for mothers and prematurity and low birth weight for babies.

Among other findings in the new CDC report:

• The birth rate in the United States in 2014 was the highest it has been since 2007.

The rate of 62.9 births for every 1,000 women in 2014 is one of the lowest on record, but it represents a 1% increase from 2013 and the first time the rate has risen between years since 2007. The report attributed the increase to greater numbers of births among women in their 30s and early 40s.

• The birth rate for women in their early and late 30s climbed 3% between 2013 and 2014.

The rates rose from 100.8 births in their early 30s and 51 births in their late 30s per 1,000 women. Among women in their early 40s, the birth rate rose 2% since 2013 to 10.6 per 1,000 women. The swelling birth rate among these age groups is part of an ongoing trend, Brady E. Hamilton, a statistician and demographer at the National Center for Health Statistics and lead author of the study, said in response to the preliminary 2014 reportreleased in June.

• There was a 9% decrease in the birth rate among teen mothers ages 15 to 19 since 2013.

The birth rate among this age group dropped to 24.2 births per 1,000, a "phenomenal" change, Hamilton said. In fact, it's an all-time low. The birth rate among women in their early 20s also decreased by 2% to 79 per 1,000, whereas the rate for women in their late 20s climbed 2% to 105.8 births per 1,000 women. Trends among men mirrored those of women, with an increase in the number of men in their 30s and 40s fathering babies and a decrease in the birth rate among men in their teens and 20s.

• White women saw the largest overall increase in birth rate -- of 1% -- between 2013 and 2014.

The birth rate dropped by 1% among Hispanic women and did not change among black women. Even with the overall uptick, the birth rate in the United States did not reach "replacement" level, or the number of births that would be needed to replace the previous generation.

• Several other trends among women continued between 2013 and 2014.

The rate among unmarried women giving birth fell for the sixth year in a row, and was 43.9 per 1,000 unmarried women in 2014.

The rate of cesarean delivery dropped for the second year in a row to 32.2% of all births in 2014, the lowest since 2007. The rate of preterm births has been on a downward trajectory since 2007, and made up 10% of births in 2014.